In machines for packaging products fed one after another which use continuous plastic film of heat-shrinkable or non-heat shrinkable type, it is known to weld the film, when wrapped around the product, in a direction transverse to the product and film feed direction, to form a package which is at least partly closed.
In this respect, it should be noted that the longitudinal superposed edges of said film always remain free, the result being that the formed package can to some extent be tampered with.
In view of this it is advantageous to weld along the longitudinal superposed edges in order to completely close the package and make it impossible to tamper with the product contained within the film.
This further longitudinal welding operation requires re-handling of the transversely welded packages, so increasing packaging costs and sometimes requiring a further operator to carry out this latter operation.
Longitudinal welding apparatus operating continuously and automatically are also known and used for this purpose. These apparatus weld the edges of the film during its advancement, by means of a welder comprising a circular or simply circumferential arc-shaped surface arranged to press on the film. In such cases the welder is mounted on a shaft so that it can undergo rotary or rocking movement to weld the film edges.
By using such apparatus it is also possible to preset the welding temperature and so adapt the temperature to the characteristics and thickness of the plastics film.
However, modern product packaging machines comprise systems for automatically controlling the speed at which the products are fed, in order to adapt the operation of the packaging machine to the rate at which the storage containers release the products to be packaged.
This results in sudden changes in the rate of product supply, such variations directly influencing the time for which the welder remains in contact with the film to be welded.
These rate variations are not previously predictable, with the result that the facility on said apparatus for presetting only the welding temperature without being able to adapt it to the rate of operation of the packaging machine is totally inadequate.
In addition the welding temperature control is difficult to achieve with the rapidity necessary for the particular type of application.
In this respect, although the temperature of a welding element can be raised rapidly by merely increasing the electrical voltage, its temperature cannot be lowered with the same rapidity.
Because of the large heat capacity of the welding element it remains at high temperature even when the voltage is removed. The rate at which the temperature falls depends totally on the heat capacity of the welding element and the rapidity with which said welding element transfers heat to the surrounding environment.
The object of the present invention is to provide a longitudinal welding apparatus in a packaging machine of the stated type and for the stated applications which enables continuous and completely automatic welding to be effected by achieving proper temperature control even during sudden variations in the rate of operation of the packaging machine.